The present invention relates to AC generators and, more particularly, to AC generators that are driven by an aircraft engine to provide a constant frequency output.
In many of today's electric power systems for aircraft, electrical generators are required to produce power at a constant frequency (e.g., 400 Hz). In some instances, constant frequency power may be produced by driving an AC generator at a constant rotational speed. For example, an auxiliary power unit (APU) may be employed to drive the AC generator. The APU may be controlled to rotate at one particular speed because its speed of operation may be independent of varying main engine power demands needed to control propulsion of the aircraft.
Alternatively some aircraft designs employ a hydromechanical transmission interposed between a variable speed engine and a generator. The transmission may be controlled so that rotational speed of the generator may remain relatively constant even though the engine speed may vary to meet propulsion demands.
In still other aircraft power systems, constant frequency AC power may be produced by coupling a constant frequency generator (CFG) to a variable speed engine. In a typical CFG, an exciter generator may supply AC power to a main generator rotor. A controller may be employed to vary speed of rotation of rotor flux so that irrespective of physical rotation speed of the rotor, AC power at the desired constant frequency is produced. In some instances, the exciter generators may be large and heavy and may contribute substantially to overall weight and cost of a CFG.
As aircraft designs evolve there is a continuous recognition of the desirability of reducing weight and cost of components. Additionally there is a continuing effort to improve engine operating parameters to reduce fuel consumption.
As can be seen, there is a need for an aircraft electrical system that may produce constant frequency AC power without incurring weight and cost penalties associated with hydromechanical transmissions, or requirements for constant speed operation of an APU. Additionally there is a need for a CFG with an exciter generator that may be constructed with relatively low weight and cost.